<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
    let count = 0;
    //开启定时器
    setInterval(f, 1000);//1000ms调用f()方法一次
    function f() {
        count++;
        let h = document.querySelector("h1");
        h.innerText = count;
    }
    //匿名方法开启定时器
    let timer = setInterval(function (){
        console.log(count);
        if (count==5)
            clearInterval(timer);
    },1000);
    //只执行一次定时器
    setTimeout(function (){
        console.log("时间到!")
    },3000);
</script>
</body>
</html>